gentree: use git library to speed up --git-revision

gentree: use git library to speed up --git-revision

If the git library is available (on many distros, just
install python-git or similar) then use it to read the
git blobs from disk if --git-revision is used.

On my system (with an SSD) this speeds up the entire
backport generation process significantly (when using
--git-revision):

before:
real	0m39.280s
user	0m13.464s
sys	0m16.644s

after:
real	0m22.554s
user	0m16.404s
sys	0m5.724s

Signed-off-by: Johannes Berg <johannes.berg@intel.com>